Electroluminescent device, manufacturing method thereof, and display device comprising the same

ABSTRACT

An electroluminescent device, a method of manufacturing the same, and a display device including the same. The electroluminescent device includes a first electrode and a second electrode facing each other; an emission layer disposed between the first electrode and the second electrode, the emission layer including light emitting particles; an electron transport layer disposed between the first electrode and the emission layer; and a hole transport layer disposed between the second electrode and the emission layer, wherein the electron transport layer includes inorganic oxide particles and a metal-organic compound, the metal-organic compound or a thermal decomposition product of the metal-organic compound being soluble a non-polar solvent.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2018-0029942 filed in the Korean IntellectualProperty Office on Mar. 14, 2018, and Korean Patent Application No.10-2019-0028358 filed in the Korean Intellectual Property Office on Mar.12, 2019, and all the benefits accruing therefrom under 35 U.S.C. § 119,the entire content of which is incorporated herein by reference.

BACKGROUND 1. Field

The present invention relates to an electroluminescent device,manufacturing method thereof, and a display device including the same.

2. Description of the Related Art

Quantum dots are a nanocrystal semiconductor material having a diameterof several to several hundreds of nanometers (nm), which exhibitsquantum confinement effects. Quantum dots generate stronger, e.g.,brighter, light in a narrow wavelength region than phosphor. Quantumdots emit light while the excited electrons are transitioned from aconduction band to a valance band and wavelengths are changed dependingupon a particle size of the quantum dot. As quantum dots emit shorterwavelength light with decreasing particle size, quantum dots may providelight in a desirable wavelength region by adjusting the sizes of thequantum dots.

In other words, the emission layer including quantum dots and thevarious electronic devices including the same may reduce productioncosts, compared with the organic light emitting diode using the emissionlayer including phosphorescence and/or phosphor material, and thedesirable color may be emitted by changing sizes of quantum dots,without use of other organic materials in the emission layer foremitting other color lights.

Luminous efficiency of the emission layer including quantum dots isdetermined by quantum efficiency of the quantum dots, a balance ofcharge carriers, light extraction efficiency, leakage current, and thelike. That is, in order to improve the luminous efficiency of theemission layer, methods, such as adjusting excitons to be confined tothe emission layer, adjusting holes and electrons to be smoothlytransported to the quantum dots, or preventing leakage currents, may beused.

SUMMARY

An electroluminescent device having improved device characteristics bydecreasing a leakage current and a display device including the same areprovided.

According to an embodiment, an electroluminescent device includes afirst electrode and a second electrode facing each other; an emissionlayer disposed between the first electrode and the second electrode, theemission layer including light emitting particles; an electron transportlayer disposed between the first electrode and the emission layer; and ahole transport layer disposed between the second electrode and theemission layer, wherein the electron transport layer includes inorganicoxide particles and a metal-organic compound, the metal-organic compoundor a thermal decomposition product of the metal-organic compound beingsoluble in a non-polar solvent.

The metal-organic compound may include Zn, Ti, Sn, W, Ta, Mg, Ga, Na,Cu, Ag, or a combination thereof.

The metal-organic compound may include an oleate compound, an aminecompound, or a combination thereof.

The non-polar solvent may include a C6 to C20 linear, branched, orcyclic aliphatic hydrocarbon, a C6 to C20 aromatic hydrocarbon,methylene chloride, ethyl acetate, or a combination thereof.

The inorganic oxide particles may include zinc, titanium, zirconium,tin, tungsten, tantalum, magnesium, gallium, sodium, copper, silver, ora combination thereof.

The inorganic oxide particles may are dispersible in a polar solvent.

A surface of the electron transport layer facing the emission layer mayinclude a first portion in which the inorganic oxide particles aredisposed and a second portion in which the metal-organic compound or thethermal decomposition product of the metal-organic compound is betweenneighboring inorganic oxide particles.

The electron transport layer may include a first layer including theinorganic oxide particles and a second layer including the metal-organiccompound or the thermal decomposition product of the metal-organiccompound.

The second layer may be disposed between the first layer and theemission layer.

The first layer may include a cluster layer including the inorganicoxide particles.

A surface of the first layer facing the second layer may include themetal-organic compound or the thermal decomposition product of themetal- organic compound.

The first layer may include an organic matrix filled in betweenneighboring inorganic oxide particles and the organic matrix may includean organic material that is different from the metal-organic compound orthe thermal decomposition product of the metal-organic compound.

An average thickness of the second layer may be less than an averagethickness of the first layer.

The inorganic oxide particles may include metal oxide particles, and ametal in the metal oxide particles and a metal in the metal-organiccompound are the same.

An average thickness of the electron transport layer may be about 20nanometers (nm) to about 60 nm.

The light emitting particles may include quantum dots.

The light emitting particles may have a core-shell structure.

The light emitting particles may include a Group II-VI compound thatdoes not include Cd, a Group III-V compound, a Group IV-VI compound, aGroup IV element or compound, a Group I-III-VI compound, a GroupI-II-IV-VI compound that does not include Cd, or a combination thereof.

A hydrophobic organic ligand may be attached to a surface of the lightemitting particles.

On the other hand, a display device including the electroluminescentdevice according to an embodiment is provided.

An electroluminescent device having improved device characteristics bydecreasing the leakage current may be provided.

In addition, as described above, a display device including theelectroluminescent device having improved device characteristics isprovided.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other advantages and features of this disclosure willbecome more apparent by describing in further detail exemplaryembodiments thereof with reference to the accompanying drawings, inwhich:

FIG. 1 is a schematic cross-sectional view of an electroluminescentdevice according to an embodiment,

FIGS. 2 to 4 are cross-sectional views illustrating various positionalrelationships between inorganic oxide particles, a metal-organiccompound, and a thermal decomposition product thereof in an electrontransport layer of an electroluminescent device according to anembodiment,

FIG. 5 shows a scanning electron microscopic (SEM) image of the surfaceof the ZnO cluster layer of Comparative Example 1,

FIG. 6 is a SEM image of the surface of a zinc-oleate or a thermaldecomposition product layer thereof formed on the ZnO cluster layer ofExample 1,

FIG. 7 is a graph of current density (milliamperes per square centimeter(mA/cm²)) versus voltage (volts (V)) of the electroluminescent devicesof Example 1 and Comparative Examples 1 to 4,

FIG. 8 is a graph of external quantum efficiency (EQE (percent (%)))versus luminescence (candela per square meter (Cd/m²)) of theelectroluminescent devices of Example 1 and Comparative Examples 1 to 4,

FIG. 9 is a graph of luminance (%) versus hours showing life-spancharacteristics of the electroluminescent devices of Example 1 andComparative Example 1, and

FIG. 10 is a graph of luminance (%) versus hours showing life-spancharacteristics of the electroluminescent devices of Example 2 andComparative Example 5.

DETAILED DESCRIPTION

Hereinafter, example embodiments will be described in detail so that aperson skilled in the art would understand the same. This disclosuremay, however, be embodied in many different forms and is not construedas limited to the example embodiments set forth herein.

In the drawings, the thickness of layers, films, panels, regions, etc.,are exaggerated for clarity. Like reference numerals designate likeelements throughout the specification. It will be understood that whenan element such as a layer, film, region, or substrate is referred to asbeing “on” another element, it can be directly on the other element orintervening elements may also be present. In contrast, when an elementis referred to as being “directly on” another element, there are nointervening elements present.

It will be understood that, although the terms “first,” “second,”“third” etc. may be used herein to describe various elements,components, regions, layers and/or sections, these elements, components,regions, layers and/or sections should not be limited by these terms.These terms are only used to distinguish one element, component, region,layer or section from another element, component, region, layer orsection. Thus, “a first element,” “component,” “region,” “layer” or“section” discussed below could be termed a second element, component,region, layer or section without departing from the teachings herein.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “At least one” is not to be construed as limiting “a” or“an.” “Or” means “and/or.” As used herein, the term “and/or” includesany and all combinations of one or more of the associated listed items.It will be further understood that the terms “comprises” and/or“comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof.

Furthermore, relative terms, such as “upper,” may be used herein todescribe one element's relationship to another element as illustrated inthe Figures. It will be understood that relative terms are intended toencompass different orientations of the device in addition to theorientation depicted in the Figures. For example, if the device in oneof the figures is turned over, elements described as being on the“lower” side of other elements would then be oriented on “upper” sidesof the other elements. The exemplary term “lower,” can therefore,encompasses both an orientation of “lower” and “upper,” depending on theparticular orientation of the figure.

“About” as used herein is inclusive of the stated value and means withinan acceptable range of deviation for the particular value as determinedby one of ordinary skill in the art, considering the measurement inquestion and the error associated with measurement of the particularquantity (i.e., the limitations of the measurement system). For example,“about” can mean within one or more standard deviations, or within ±30%,20%, 10% or 5% of the stated value.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

Exemplary embodiments are described herein with reference to crosssection illustrations that are schematic illustrations of idealizedembodiments. As such, variations from the shapes of the illustrations asa result, for example, of manufacturing techniques and/or tolerances,are to be expected. Thus, embodiments described herein should not beconstrued as limited to the particular shapes of regions as illustratedherein but are to include deviations in shapes that result, for example,from manufacturing. For example, a region illustrated or described asflat may, typically, have rough and/or nonlinear features. Moreover,sharp angles that are illustrated may be rounded. Thus, the regionsillustrated in the figures are schematic in nature and their shapes arenot intended to illustrate the precise shape of a region and are notintended to limit the scope of the present claims.

As used herein, “group” may refer to a group of Periodic Table.

As used herein, “Group II” may refer to Group IIA and Group IIB, andexamples of Group II metal may be Cd, Zn, Hg, and Mg, but are notlimited thereto.

As used herein, examples of “Group II metal that does not include Cd”may refer to a Group II metal except Cd, for example, Zn, Hg, Mg, etc.

As used herein, “Group Ill” may refer to Group IIIA and Group IIIB, andexamples of Group III metal may be Al, In, Ga, and TI, but are notlimited thereto.

As used herein, “Group IV” may refer to Group IVA and Group IVB, andexamples of a Group IV metal may be Si, Ge, and Sn, but are not limitedthereto. As used herein, the term “metal” may include a semi-metal suchas Si.

As used herein, “Group I” may refer to Group IA and Group IB, andexamples may include Li, Na, K, Rb, and Cs, but are not limited thereto.

As used herein, “Group V” may refer to Group VA, and examples mayinclude nitrogen, phosphorus, arsenic, antimony, and bismuth, but arenot limited thereto.

As used herein, “Group VI” may refer to Group VIA, and examples mayinclude sulfur, selenium, and tellurium, but are not limited thereto.

As used herein, “aliphatic” means a saturated or unsaturated linear orbranched hydrocarbon group. An aliphatic group may be an alkyl, alkenyl,or alkynyl group, for example.

As used herein, an “amine” has the general formula NRR, wherein each Ris independently hydrogen, a C1-030 alkyl group, a C3-C8 cycloalkylgroup a C2-C30 alkenyl group, a C2-C30 alkynyl group, or a C6-C30 arylgroup, each of which may be substituted or unsubstituted.

As used herein, “aromatic” means an organic compound or group comprisingat least one unsaturated cyclic group having delocalized pi electrons.The term encompasses both hydrocarbon aromatic compounds andheteroaromatic compounds.

In an embodiment, for particle diameters of particles, although theparticle diameters may be quantitated by a measurement to show anaverage size of a group, the method may include a mode diameter showingthe maximum value of the distribution, a median diameter correspondingto the center value of integral distribution curve, a variety of averagediameters (numeral average, length average, area average, mass average,volume average, etc.), and the like. Unless particularly mentionedotherwise, “average particle diameters” means numeral average diametersin the present disclosure, and the average particle diameter is obtainedby measuring D50 (particle diameters at a position of distribution rateof 50%).

In an embodiment, “solubility” indicates property that a solute is welldissolved in a particular solvent. In an embodiment, when a solute iswell dissolved in a non-polar solvent, the corresponding solute may beexpressed as having a solubility regarding, e.g., in, the non-polarsolvent, while a solute is well dissolved in a polar solvent, thecorresponding solute may be expressed as having a solubility regarding,e.g., in, the polar solvent.

Unless otherwise described in an embodiment, a thickness of eachconstituent element may refer to “an average thickness.”

The “average thickness” of each constituent element is calculated byextracting arbitrary 5 to 10 points from a scanning electron microscopic(SEM) cross-sectional image of each constituent element, measuringthicknesses, and calculating an arithmetic average of the measuredthicknesses.

FIG. 1 is a schematic cross-sectional view of an electroluminescentdevice according to an embodiment and FIGS. 2 to 4 are cross-sectionalviews illustrating various positional relationships between inorganicoxide particles, a metal-organic compound, and a thermal decompositionproduct thereof in an electron transport layer of an electroluminescentdevice according to an embodiment.

An electroluminescent device 10 according to an embodiment includes afirst electrode 110 and a second electrode 170 facing each other, anemission layer 141 disposed between the first electrode 110 and thesecond electrode 170 and including light emitting particles 141; anelectron transport layer 120 disposed between the first electrode 110and the emission layer 140; and a hole transport layer 150 disposedbetween the second electrode 170 and the emission layer 140, and a holeinjection layer 160 that is disposed between the hole transport layer150 and the second electrode 170 and may be omitted in consideration ofa relationship of each constituent element.

The electroluminescent device 10 may have a laminate structure where theelectron transport layer 120, the emission layer 140, the hole transportlayer 150, and the hole injection layer 160 are disposed between thefirst electrode 110 and the second electrode 170 facing each other.

The electroluminescent device 10 according to an embodiment suppliescurrent to the emission layer 140 through the first electrode 110 andthe second electrode 170 and causes electroluminescence of the lightemitting particles 141 to generate light. The electroluminescent device10 may generate light in various wavelength regions according tomaterials, sizes, or fine structures of the light emitting particles 141of the emission layer 140.

In an embodiment, the first electrode 110 and the second electrode 170may be respectively connected to a driving power source and may functionto supply current to the emission layer 140. One of the first electrode110 and the second electrode 170 may be an anode and the other may be acathode. In an embodiment, the first electrode 110 may be a cathode andthe second electrode 170 may be an anode.

At least one of the first electrode 110 and the second electrode 170 mayinclude a material having light transmittance in at least visible lightwavelength region but is not limited thereto. The first electrode 110may include a material having light transmittance in an infrared orultraviolet (UV) wavelength region. For example, the first electrode 110may include an optically transparent material. For example, the secondelectrode 170 may include an optically transparent material. Forexample, the first electrode 110 and the second electrode 170 mayinclude an optically transparent material.

In an embodiment, the optically transparent material may include, forexample, molybdenum oxide, tungsten oxide, vanadium oxide, rheniumoxide, niobium oxide, tantalum oxide, titanium oxide, zinc oxide, nickeloxide, copper oxide, cobalt oxide, manganese oxide, chromium oxide,indium oxide, tin oxide, indium tin oxide, indium zinc oxide, or acombination thereof, or silver (Ag), aluminum (Al), copper (Cu), gold(Au), an alloy thereof, or a combination thereof.

In an embodiment, one of the first electrode 110 and the secondelectrode 170 may include molybdenum oxide, tungsten oxide, vanadiumoxide, rhenium oxide, niobium oxide, tantalum oxide, titanium oxide,zinc oxide, nickel oxide, copper oxide, cobalt oxide, manganese oxide,chromium oxide, indium oxide, tin oxide, indium tin oxide, indium zincoxide, or a combination thereof and the other of the first electrode 110and the second electrode 170 may include silver (Ag), aluminum (Al),copper (Cu), gold (Au), an alloy thereof, or a combination thereof. Forexample, the first electrode 110 may include molybdenum oxide, tungstenoxide, vanadium oxide, rhenium oxide, niobium oxide, tantalum oxide,titanium oxide, zinc oxide, nickel oxide, copper oxide, cobalt oxide,manganese oxide, chromium oxide, indium oxide, tin oxide, indium tinoxide, indium zinc oxide, or a combination thereof and the secondelectrode 170 may include silver (Ag), aluminum (Al), copper (Cu), gold(Au), an alloy thereof, or a combination thereof.

However, the first electrode 110 and/or the second electrode 170according to an embodiment are not necessarily limited thereto but mayinclude a material further having light transmittance with respect tolight in an infrared or ultraviolet (UV) wavelength region or asemi-permeable material selectively transmitting light in a particularwavelength region and may conduct a function of reflecting light in avisible light wavelength region.

Each of the first electrode 110 and the second electrode 170 may beformed by depositing a material for forming an electrode on thesubstrate 100 or an organic layer by a method such as sputtering ordeposition.

Meanwhile, in an embodiment, the first electrode 110 may be disposed onthe substrate 100 as shown in FIG. 1. The substrate 100 may be atransparent insulating substrate or may include a ductile material. Thesubstrate 100 may include glass or a polymer material, and the polymermaterial may have, for example, a glass transition temperature (Tg) ofgreater than or equal to about 150° C. For example, the substrate 100may include a COC (cycloolefin copolymer) or COP (cycloolefin polymer)based material.

In an embodiment, the substrate 100 may support the electron transportlayer 120, the emission layer 140, the hole transport layer 150, and thehole injection layer 160 disposed between the first electrode 110 andthe second electrode 170

However, the substrate 100 of the electroluminescent device 10 accordingto an embodiment may not be disposed under the first electrode 110, butthe substrate 100 of the electroluminescent device 10 may be disposed onthe second electrode 170 or may be omitted, as desired.

In an embodiment, the electron transport layer 120 is disposed betweenthe first electrode 110 and the emission layer 140 and transportscharges from the first electrode 110 into the emission layer 140. Thecharges may be electrons.

In an embodiment, an average thickness of the electron transport layer120 may be variously changed in consideration of a charge carrierbalance of the hole transport layer 150, the hole injection layer 160,the emission layer 140, or a combination thereof in the device, but maybe, for example, greater than or equal to about 20 nm, greater than orequal to about 25 nm, greater than or equal to about 30 nm, greater thanor equal to about 35 nm, or greater than or equal to about 40 nm, and,for example, less than or equal to about 100 nm, less than or equal toabout 90 nm, less than or equal to about 80 nm, less than or equal toabout 70 nm, less than or equal to about 60 nm, less than or equal toabout 55 nm, or less than or equal to about 50 nm, for example, about 20nm to about 60 nm, about 30 nm to about 60 nm, or about 40 nm to about60 nm.

When the average thickness of the electron transport layer 120 is lessthan about 20 nm, voids, cracks, and the like of the electron transportlayer 120 may be increased and the device characteristics may bedeteriorated, and it may be difficult to meet, e.g., achieve, a carrierbalance with other constituent elements of the electroluminescentdevice.

On the other hand, when the average thickness of the electron transportlayer 120 is greater than about 100 nm, electrons may be supplied to theemission layer 140 too fast, in too great a quantity, or a combinationthereof compared with holes, and electrons may meet holes on theinterface between the emission layer 140 and the hole transport layer150 to cause interfacial light emission or may move to the holetransport layer 150, the hole injection layer 160, ora combinationthereof to be quenched.

In an embodiment, the electron transport layer 120 may include inorganicoxide particles 121. For example, the electron transport layer 120 mayinclude a cluster layer composed of the inorganic oxide particles 121.

In an embodiment, the inorganic oxide particles 121 may be composed ofmaterials having electron transporting properties and thus the inorganicoxide particles 121 may have electron transporting properties. Theinorganic oxide particles 121 according to an embodiment may be, forexample, oxides including zinc, titanium, zirconium, tin, tungsten,tantalum, magnesium, gallium, sodium, copper, silver, or a combinationthereof and may include, for example, ZnO, TiO₂, ZrO₂, SnO₂, WO₃, Ta₂O₃,or a combination thereof, but are not limited thereto.

The inorganic oxide particles 121 according to an embodiment may havedispersibility with respect to, e.g., be dispersible in, a polarsolvent. The dispersibility of the inorganic oxide particles 121 may becaused by the hydroxyl group (—OH) on the surface of the inorganic oxideparticles 121 at room temperature. For example, ZnO particles mayinclude a large numbers of hydroxyl groups (—OH) on the surface at roomtemperature. Therefore, when the ZnO particles are introduced into apolar solvent such as water or ethanol at room temperature, the ZnOparticles may be controlled to be dispersed evenly in the polar solventwithout being agglomerated by a relatively simple physical treatmentsuch as stirring.

Accordingly, it may be relatively easy to form a cluster layer composedof the inorganic oxide particles 121 by applying a solution in which theinorganic oxide particles 121 are dispersed in a polar solvent on thefirst electrode 110 and then drying the solution. In addition, thecluster layer formed thereon may have a relatively uniform distributionof the inorganic oxide particles 121 without agglomerating at a specificportion.

On the other hand, an average particle diameter of the inorganic oxideparticles 121 according to an embodiment may be, for example, less thanor equal to about 70 nm, less than or equal to about 60 nm, less than orequal to about 50 nm, less than or equal to about 40 nm, less than orequal to about 30 nm, less than or equal to about 20 nm, or less than orequal to about 10 nm, and, for example, greater than or equal to about 1nm, greater than or equal to about 2 nm, greater than or equal to about3 nm, greater than or equal to about 4 nm, or greater than or equal toabout 5 nm.

When the average particle diameter of the inorganic oxide particles 121is within the above ranges, the cluster layer composed of the inorganicoxide particles 121 may exhibit excellent electron transportingproperties.

In an embodiment, the electron transport layer 120 may include ametal-organic compound or a thermal decomposition product thereof havingsolubility with respect to, e.g., being soluble in, a non-polar solvent.

The metal-organic compound may be a metal salt having an organic groupand may be, for example, a composite of a metal cation and an organicanion. The metal-organic compound has electron transporting propertieslike the above-mentioned inorganic oxide particles 121 but may havesolubility with respect to a non-polar solvent, unlike the inorganicoxide particles 121.

In an embodiment, the metal-organic compound may have the same or asimilar level of electron transporting properties compared with theinorganic oxide particles 121. The metal in the metal-organic compoundmay be a metal of Zn, Ti, Sn, W, Ta, Mg, Ga, Na, Cu, Ag, or acombination thereof.

On the other hand, the electron mobility and energy level of themetal-organic compound may vary depending on the metal, organiccompound, or a combination thereof, but the electron mobility, theenergy level, or a combination thereof may be the same as or similar tothose of the inorganic oxide particles 121.

For example, the metal in the metal-organic compound may be the same asthe metal in the inorganic oxide particles 121. For example, theinorganic oxide particles 121 may be metal oxide particles, and themetal element included in the metal oxide particles and the metalelement contained in the metal-organic compound may be the same eachother. In this case, it may be relatively easy to control the energylevel, the electron mobility, or a combination thereof of the electrontransport layer 120 to the same or a similar level.

However, an embodiment is not necessarily limited thereto, and the metalin the inorganic oxide particles 121 and the metal of the metal-organiccompound may be different from each other in consideration of a chargecarrier balances between the respective constituent elements of theelectroluminescent element 10.

In an embodiment, the organic compound in the metal-organic compoundimparts solubility with respect to the non-polar solvent to themetal-organic compound. The organic compound may include an oleate-basedcompound, an amine-based compound, or a combination thereof. Forexample, the organic compound may be an oleate-based compound. However,an embodiment is not limited thereto, and the organic compound mayinclude an oleate-based compound, an amine-based compound, a combinationthereof, or at least other organic compounds that are different from theoleate-based compound or the amine-based compound.

In addition, examples of the non-polar solvent for dissolving themetal-organic compound may a C6 to C20 linear, branched, or cyclicaliphatic hydrocarbon, C6 to C20 aromatic hydrocarbon, methylenechloride, ethyl acetate, or a combination thereof. The C6 to C20 linear,branched, or cyclic aliphatic hydrocarbon may contain one or moreunsaturated groups, but is not aromatic. Heteroatoms may or may not bepresent in the aliphatic or aromatic hydrocarbons, and are only presentprovided that the aliphatic or aromatic hydrocarbons are nonpolar.

When the organic compound in the metal-organic compound according to anembodiment is as disclosed above, a solution containing a metal-organiccompound in a non-polar solvent may be applied to the cluster layer andthen dried to form an electron transport layer 120 in a relativelysimple manner.

Specifically, since the above-described cluster layer is produced usinga polar solvent, when the metal-organic compound having solubility withrespect to a polar solvent is used, the cluster layer may be damaged bythe polar solvent. However, since the metal-organic compound accordingto an embodiment uses a non-polar solvent having chemicalcharacteristics different from those of the inorganic oxide particles121 damage to the surface of the cluster layer may be minimized when themetal-organic compound is applied on the cluster layer.

The thermal decomposition product of the metal-organic compound mayrefer to a product obtained by thermally decomposing a solutionincluding the metal-organic compound in a process of coating and dryingat a predetermined temperature. The thermal decomposition product of themetal-organic compound may include a metal, metal oxide, an organiccompound, or a combination thereof.

Specifically, the thermal decomposition product may include a metal ofZn, Ti, Sn, W, Ta, Mg, Ga, Na, Cu, Ag, or a combination thereof, oroxides thereof, and carbon compounds.

The carbon compound may be a product in which the organic compound inthe metal-organic compound, specifically, the oleate compound, the aminecompound, and other organic compounds are separated from the metal bythe thermal decomposition of the metal-organic compound, or a derivativethereof. The thermal decomposition product may be chemically bound tothe surface of the inorganic oxide particles 121.

The carbon compound may have solubility with respect to a non-polarsolvent.

When the metal-organic compound or the thermal decomposition productthereof are included together with the inorganic oxide particles 121 asin the embodiment, there is a difference in an energy level changecompared with the case where the inorganic oxide particles 121 are dopedwith metal.

Specifically, when Mg is used as a metal in the metal-organic compoundand ZnO particles are used as the inorganic oxide particles 121 theelectron transport layer 120 in which the pure Mg-organic compound orthe thermal decomposition product thereof and the ZnO particles aremixed has slightly reduced lowest unoccupied molecular orbital (LUMO)energy and highest occupied molecular orbital (HOMO) energy levelscompared with the electron transport layer including the ZnO particlesalone. On the other hand, ZnMgO in which ZnO is doped with Mg mayexhibit slightly increased LUMO and HOMO energy levels compared with theelectron transport layer including ZnO particles alone.

In an embodiment, amounts of the metal-organic compound may be changedvariously by regarding the relationship between the above-describedenergy level, electron mobility. The amounts of the metal-organiccompound may be variously adjusted in consideration of, for example, theorganic compound bonding with the metal, the inorganic oxide particles121 thicknesses of the electron transport layer 120, and the like.

Referring to FIGS. 2 to 4, the electron transport layer 120 in theelectroluminescent devices 10, 10′, and 10″ may include a first layerincluding a cluster layer composed of the inorganic oxide particles 121.In this case, the metal-organic compound or the thermal decompositionproduct thereof may be disposed at various positions between the firstlayer 124 and the emission layer 140.

The average thickness of the first layer 124 may be changed inconsideration of average particle diameters and materials of theinorganic oxide particles 121 and charge carrier balances between theemission layer 140, the hole transport layer 150, or a combinationthereof, the hole injection layer 160, and the like. However, theaverage thickness of the first layer 124 may be, for example, less thanor equal to about 60 nm, less than or equal to about 50 nm, less than orequal to about 40 nm, less than or equal to about 35 nm, or less than orequal to about 30 nm, and, for example, greater than or equal to about20 nm, greater than or equal to about 25 nm, or greater than or equal toabout 30 nm.

When the average thickness of the first layer 124 satisfies the aboveranges, the first layer 124 may exhibit excellent electron transportingproperties.

Referring to FIG. 2, a metal-organic compound or a thermal decompositionproduct thereof 122 in the electroluminescent device 10 directlycontacts the surface of the first layer 124 facing the emission layer140 and also may fill an empty space on the surface.

Specifically, with reference to the surface of the electron transportlayer 120 facing the emission layer 140, the surface may include a firstregion in which the inorganic oxide particles 121 is present and asecond region in which the metal-organic compound or the thermaldecomposition product thereof 122 filling the empty space among theneighboring inorganic oxide particles 121 is present.

On the surface of the electron transport layer 120 facing the emissionlayer 140, the inorganic oxide particles 121 may be mixed with themetal-organic compound or the thermal decomposition product thereof 122.

Accordingly, as for the electroluminescent device 10 according to FIG.2, the electron transport layer 120 may be a composite layer includingthe first layer 124 including the inorganic oxide particles 121 and alayer including the metal-organic compound or the thermal decompositionproduct thereof 122 and filling an empty space on the surface of thefirst layer 124 facing an emission layer, as described above.

On the other hand, referring to FIG. 3, an electroluminescent device 10′may further include a second layer 126 having a predetermined thicknessbetween the emission layer 140 and the first layer 124 unlike theelectroluminescent device 10 according to FIG. 2, wherein the secondlayer 126 may include a metal-organic compound or a thermaldecomposition product thereof. For example, the second layer 126 may bea layer formed of the metal-organic compound or the thermaldecomposition product thereof.

Specifically, the surface of the electron transport layer 120 facing theemission layer 140 may be the surface of the second layer 126, and theinorganic oxide particles 121 may not be exposed to the surface butcovered by the second layer 126 unlike the electroluminescent device 10according to FIG. 2. The surface of the second layer 126 facing theemission layer 140 may be relatively smooth compared with the surface ofthe first layer 124 contacting the second layer 126. Accordingly,morphology of the electron transport layer 120 contacting the emissionlayer 140 may be improved.

As for the electroluminescent device 10′ of FIG. 3 like theelectroluminescent device 10 of FIG. 2, the surface of the first layer124 directly contacts the metal-organic compound or the thermaldecomposition product thereof, which may fill an empty space on thesurface of the first layer 124.

The second layer 126 according to an embodiment has a thickness enoughto at least cover the surface of the first layer 124 contacting itselfand not to expose the inorganic oxide particles 121 toward the emissionlayer 140, as described above. On the other hand, the second layer 126may be formed to be thicker than a desired thickness in consideration ofinteraction with a solution for forming the emission layer 140 duringformation of the emission layer 140 which will be described later.

In an embodiment, the second layer 126 may have a thinner averagethickness than that of the first layer 120 in consideration of all theabove conditions, for example, an average thickness of greater than orequal to about 2 nm, greater than or equal to about 3 nm, greater thanor equal to about 4 nm, greater than or equal to about 5 nm, or greaterthan or equal to about 10 nm, and, for example, less than or equal toabout 15 nm, for example, in a range of about 3 nm to about 15 nm, forexample, in a range of about 5 nm to about 15 nm, for example, in arange of about 7 nm to about 15 nm, or for example, in a range of about10 nm to about 15 nm.

When the second layer 126 has an average thickness of less than about 2nm, the thickness of the second layer 126 may be decreased by anon-polar solvent during formation of the emission layer 140 which willbe described later, and the inorganic oxide particles 121 may beexposed. On the other hand, when the second layer 126 has an averagethickness of greater than about 15 nm, a charge carrier balance of thewhole electroluminescent device may be deteriorated.

In the electroluminescent devices 10 and 10′ according to FIGS. 2 and 3,the empty space on the surface of the first layer 124 may be changeddepending on an average particle diameter of the inorganic oxideparticles 121 the thickness of the first layer 124, and the like butinevitably formed, since the first layer 124 is a cluster layer formedof the inorganic oxide particles 121. The empty space includes a void, acrack, or the like among the inorganic oxide particles 121.

The empty space may function as a leakage path of electrons duringoperation of a device and may not be completely removed, though thefirst layer 124 may be formed by using inorganic oxide having excellentdispersibility regarding, e.g., in, a polar solvent.

However, in the electroluminescent devices 10 and 10′ of FIGS. 2 and 3,the empty space of the first layer 124 may not only be filled with ametal-organic compound or a thermal decomposition product thereof, butalso the surface of the first layer 124 may be covered by the secondlayer 126 as shown in the electroluminescent device 10′ of FIG. 3.

In other words, as for the electroluminescent devices 10 and 10′ ofFIGS. 2 and 3, a leakage path of an electron transport layer may bereduced by filling the space of the first layer 124 with themetal-organic compound or the thermal decomposition product thereof.

However, an embodiment is not necessarily limited thereto. For example,as for an electroluminescent device 10″ of FIG. 4, a first layer 124′may include an organic material matrix filled among the, e.g., inbetween, neighboring inorganic oxide particles 121 unlike the clusterlayer of the electroluminescent devices 10 and 10′ of FIGS. 2 and 3.Herein, the organic material matrix may include an organic material of amonomer, an oligomer, a polymer, or a combination thereof, wherein theorganic material may be different from the metal-organic compound or thethermal decomposition product thereof.

For example, the organic material matrix may be a binder. In otherwords, the first layer 124′ may be a mixed layer of the inorganic oxideparticles 121 and an organic material functioning as a binder. Theorganic material matrix may include various suitable organic materialswithout being particularly limited, provided that the organic materialis different from the metal-organic compound or the thermaldecomposition product thereof and functions as a binder.

On the other hand, the second layer 126′ may not fill an empty spaceinside the first layer 124′ but cover the first layer 124′, and thesurface of the first layer 124′ may not be exposed to the emission layer140. Herein, morphology of the surface of the first layer 124′ may beimproved by covering the surface of the first layer 124′ with the secondlayer 126′, through which a leakage current of an electron transportlayer may be reduced.

In an embodiment, the electroluminescent devices 10′ and 10″ accordingto FIGS. 3 and 4 may have an equal or almost similar energy levelbetween the first layers 124 and 124′ and the second layers 126 and126′. For example, when the same metal is used for the first layers 124and 124′ and the second layers 126 and 126′, a LUMO energy level of thesecond layers 126 and 126′ may be subtly increased by an organiccompound and the like compared with that of the first layers 124 and124′.

However, a LUMO energy level difference between the second layers 126and 126′ and the first layers 124 and 124′ may be, for example, lessthan or equal to about 0.3 electronvolts (eV), less than or equal toabout 0.2 eV, or less than or equal to about 0.1 eV and may be designedto be close to 0 eV by changing a metal and an organic compound used foran electron transport layer as described above.

On the other hand, an embodiment is not necessarily limited thereto, butthe second layers 126 and 126′ and the first layers 124 and 124′ may bevariously adjusted to have a LUMO energy level difference of, forexample, greater than about 0.3 eV, for example, greater than or equalto about 0.4 eV, greater than or equal to about 0.5 eV, or, for example,in a range of about 0.3 eV to about 0.7 eV. However, thicknesses of thesecond layers 126 and 126′ and the like may be adjusted to minimize thesecond layers 126 and 126′ from working as an energy barrier.

On the other hand, between the electron transport layer 120 and thefirst electrode 110, an electron injection layer facilitating injectionof electrons, a hole blocking layer blocking movement of holes, or acombination thereof may be further formed.

Each average thickness of the electron injection layer and the holeblocking layer may be appropriately selected. For example, an averagethickness of each layer may be greater than or equal to about 1 nm andless than or equal to about 500 nm but is not limited thereto. Theelectron injection layer may be an organic layer formed by depositionand may be omitted in consideration of the thicknesses and materials ofthe electron transport layer 120.

The electron injection layer may include, for example,1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), bathocuproine(BCP), tris[3-(3-pyridyl)-mesityl]borane (3TPYMB), LiF, Alq₃, Gaq₃,Inq₃, Znq₂, Zn(BTZ)₂, BeBq₂, ET204(8-(4-(4,6-di(naphthalen-2-yl)-1,3,5-triazin-2-yl)phenyl)quinolone),8-hydroxyquinolinato lithium (Liq), an n-type metal oxide (e.g., ZnO,HfO₂, etc.), or a combination thereof, but is not limited thereto.

The hole blocking layer may include, for example,1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA), bathocuproine(BCP), tris[3-(3-pyridyl)-mesityl] borane (3TPYMB), LiF, Alq₃, Gaq₃,Inq₃, Znq₂, Zn(BTZ)₂, BeBq₂, or a combination thereof, but is notlimited thereto. The hole blocking layer may be omitted in considerationof the thicknesses, materials, and the like of other constituentelements in the electroluminescent device 10.

The emission layer 140 may be disposed between the electron transportlayer 120 and the hole transport layer 150. The emission layer 140 maybe in contact with the electron transport layer 120 and may include atleast two light emitting particles.

The emission layer 140 is a site to which electrons and holes aretransported by a current supplied from the first electrode 110 and thesecond electrode 170. The electrons and holes are combined in theemission layer 140 to generate excitons, and the generated excitons aretransitioned from an exited state to a ground state to emit light in awavelength corresponding to a size of the light emitting particles 141.

On the other hand, the emission layer 140 may emit light in apredetermined wavelength region. The predetermined wavelength regionbelongs to a visible light region, for example, one of a firstwavelength region of about 380 nm to about 488 nm, a second wavelengthregion of about 490 nm to 510 nm, a third wavelength region of greaterthan 510 nm to about 580 nm, a fourth wavelength region of about 582 nmto about 600 nm, and a fifth wavelength region of about 620 nm to about680 nm.

In an embodiment, the light emitting particles 141 may include quantumdots. That is, all of the light emitting particles 141 may be quantumdots. One of the light emitting particles 141 may be formed of quantumdots, and another of the light emitting particles 141 may be a differentlight emitting body distinguished from the quantum dot, for example, acommercially available phosphor.

The quantum dots have a discontinuous energy band gap by the quantumconfinement effect so as to convert incident light to light having apredetermined wavelength and to radiate the same. That is, when thelight emitting particles 141 are composed of quantum dots, the emissionlayer 140 may generate light having excellent color reproducibility andcolor purity.

In an embodiment, a material of the quantum dots is not particularlylimited and the quantum dots 3 may be commercially available. Forexample, the light emitting particles 141 according to an embodiment maybe quantum dots including a Group II-VI compound that does not includeCd, a Group III-V compound, a Group IV-VI compound, a Group IV elementor compound, a Group compound, a Group I-II-IV-VI compound that does notinclude Cd, or a combination thereof. That is, the light emittingparticles 141 according to an embodiment may be non-cadmium-basedquantum dots. Like this, when the light emitting particles 141 areformed of non-cadmium-cadmium-based materials, the light emittingparticles 141 may have little or no toxicity compared with aconventional cadmium-based quantum dots and thus are not dangerous andenvironmentally-friendly.

The Group II-VI compound may be a binary element compound of ZnS, ZnSe,ZnTe, ZnO, HgS, HgSe, HgTe, MgSe, MgS, or a combination thereof; aternary element compound of ZnSeS, ZnSeTe, ZnSTe, HgSeS, HgSeTe, HgSTe,HgZnS, HgZnSe, HgZnTe, MgZnSe, MgZnS, or a combination thereof;

or a quaternary element compound of ZnSeSTe, HgZnTeS, HgZnSeS, HgZnSeTe,HgZnSTe, or a combination thereof. The Group II-VI compound may furtherinclude a Group III metal.

The Group III-V compound may be a binary element compound of GaN, GaP,GaAs, GaSb, AIN, AIP, AIAs, AlSb, InN, InP, InAs, InSb, or a combinationthereof; a ternary element compound of GaNP, GaNAs, GaNSb, GaPAs, GaPSb,AINP, AINAs, AINSb, AIPAs, AIPSb, InNP, InNAs, InNSb, InPAs, InPSb,InZnP, or a combination thereof; or a quaternary element compound ofGaAINP, GaAINAs, GaAINSb, GaAIPAs, GaAIPSb, GaInNP, GaInNAs, GaInNSb,GaInPAs, GaInPSb, InAINP, InAINAs, InAINSb, InAIPAs, InAIPSb, or acombination thereof. The Group III-V compound may further include aGroup II metal (InZnP).

The Group IV-VI compound may be a binary element compound of SnS, SnSe,SnTe, PbS, PbSe, PbTe, or a combination thereof; a ternary elementcompound of SnSeS, SnSeTe, SnSTe, PbSeS, PbSeTe, PbSTe, SnPbS, SnPbSe,SnPbTe, or a combination thereof; or a quaternary element compound ofSnPbSSe, SnPbSeTe, SnPbSTe, or a combination thereof. Examples of theGroup I-III-VI compound may be CuInSe₂, CuInS₂, CuInGaSe, or CuInGaS,but are not limited thereto. Examples of the Group I-II-IV-VI compoundmay be CuZnSnSe or CuZnSnS, but are not limited thereto. Examples of theGroup IV compound may be a single substance of Si, Ge, or a combinationthereof; or a binary element compound of SiC, SiGe, or a combinationthereof.

The binary element compound, the ternary element compound or thequaternary element compound respectively exist in a uniformconcentration in the particle or in partially different concentrationsin the same particle.

According to an embodiment, the quantum dot may have a core-shellstructure including one semiconductor nanocrystal core and anothersemiconductor nanocrystal shell surrounding the core. The core and theshell may have a concentration gradient wherein the concentration of theelement(s) of the shell decreases in a direction from an outermostsurface of the quantum dot toward the core. In addition, the quantum dotmay have one semiconductor nanocrystal core and multi-shells surroundingthe core. Herein, the multi-layered shell structure has a structure oftwo or more shells and each layer may have a single composition or analloy or may have a concentration gradient.

In an embodiment, the light emitting particles may have a core-shellstructure. When the light emitting particles have a core-shellstructure, a material composition of the shell has a larger energybandgap than that of the core, which may exhibit an effective quantumconfinement effect. However, the embodiment is not limited thereto.Meanwhile, in the multi-layered shell, a shell that is outside of thecore has may have a higher energy bandgap than a shell that is near tothe core and quantum dots may have an ultraviolet (UV) to infraredwavelength ranges.

The quantum dot may have quantum efficiency of greater than or equal toabout 10%, for example, greater than or equal to about 20%, greater thanor equal to about 30%, greater than or equal to about 40%, greater thanor equal to about 50%, greater than or equal to about 60%, greater thanor equal to about 70%, greater than or equal to about 90%, or 100%.

In a display, the quantum dot may have a relatively narrow spectrum soas to improve color purity or color reproducibility. The quantum dot mayhave, for example, a full width at half maximum (FWHM) of wavelengthspectrum of less than or equal to about 45 nm, less than or equal toabout 40 nm, or less than or equal to or about 30 nm. Within the ranges,color purity or color reproducibility of a device may be improved.

The quantum dot may have a particle diameter (the longest diameter for anon-spherically shaped particle) of about 1 nm to about 100 nm). Forexample, the quantum dot may have a particle diameter of about 1 nm toabout 20 nm, for example, about 2 nm (or about 3 nm) to about 15 nm.

In addition, a shape of the quantum dot may be any suitable shape andthus may not be particularly limited. For example, the quantum dot mayhave a spherical, oval, tetrahedral, pyramidal, cuboctahedral,cylindrical, polyhedral, multi-armed, or cube nanoparticle, nanotube,nanowire, nanofiber, nanosheet, or a combination thereof. The quantumdot may have any suitable cross-sectional shape.

The quantum dot may be commercially available or may be synthesized inany suitable method. For example, several nano-sized quantum dots may besynthesized according to a wet chemical process. In the wet chemicalprocess, precursors react in an organic solvent to grow crystalparticles, and the organic solvent or a ligand compound may coordinate,e.g., become bound to, the surface of the quantum dot, controlling thegrowth of the crystal. Examples of the organic solvent and the ligandcompound may be readily determined by one skilled in the art. Theorganic solvent coordinated on the surface of the quantum dot may affectstability of a device, and thus residues that are not coordinated on thesurface of the nanocrystals may be removed by pouring the organicsolvent including the crystal in excess non-solvent and centrifuging theresulting mixture. Examples of the non-solvent may be acetone, ethanol,methanol, or the like, but are not limited thereto. After the removal ofresidues, the amount of ligand compounds, organic solvents, or acombination thereof which are coordinated on the surface of the quantumdot may be less than or equal to about 50% by weight, for example, lessthan or equal to about 30 wt %, less than or equal to about 20 wt %, orless than or equal to about 10 wt % based on a weight of the quantumdot.

A hydrophobic organic ligand having a hydrophobic moiety may be bound tothe surface of the light emitting particle. In an embodiment, thehydrophobic organic ligand may include RCOOH, RNH₂, R₂NH, R₃N, RSH,R₃PO, R₃P, ROH, RCOOR′, RPO(OH)₂, R₂POOH (wherein, R and R′ areindependently a C5 to C24 alkyl group, a C5 to C24 alkenyl group, a C5to C20 alicyclic group, or a C6 to C20 aryl group), a polymeric organicligand compound, or a combination thereof. For example, the hydrophobicorganic ligand may be oleic acid, palmitic acid, myristic acid, stearicacid, or a combination thereof. The hydrophobic organic ligand may be anorganic ligand having a mono-functional group, and the functional groupmay be chemically bound to the surface of the quantum dot.

When the hydrophobic organic ligand is attached to the surface of thelight emitting particles, according to an embodiment, both of the lightemitting body particles and the light emitting layer as a whole mayhave, e.g., exhibit, hydrophobicity. If the emissive layer 140 has,e.g., exhibits, hydrophobicity, the emissive layer 140 may be vulnerableto a non-polar solvent. That is, when the emission layer 140 and thenon-polar solvent are in contact with each other, the emission layer 140may be damaged by the non-polar solvent and thus device characteristicsmay be deteriorated.

However, the electroluminescent device 10 according to an embodiment mayinclude the aforementioned metal-organic compound or thermaldecomposition product thereof between a cluster layer formed of theinorganic oxide particles 121 and the emission layer 140 as describedabove, wherein the metal-organic compound or the thermal decompositionproduct thereof may fill an empty space on the surface of the clusterlayer and form a layer having a predetermined thickness between thecluster layer and the emission layer 140. Accordingly, when the emissionlayer 140, which is hydrophobic, is formed on the already-formedelectron transport layer 120, the emission layer may be prevented from adamage by a non-polar solvent.

The hole transport layer 150 may be disposed between the emission layer140 and the second electrode 170, between the hole injection layer 160and the emission layer 140, or between the emission layer 140 and thesecond electrode 170 and between the hole injection layer 160 and theemission layer 140. The hole transport layer 150 may provide andtransport holes into the emission layer 140. The hole transport layer150 may be formed directly under the emission layer 140 and may directlycontact the emission layer 140.

In an embodiment, the hole transport layer 150 may be formed of thep-type semiconductor material, or the material doped with a p-typedopant. For example, the hole transport layer 150 may include a PEDOT(poly(3,4-ethylenedioxythiophene)) derivative, a PSS (poly(styrenesulfonate)) derivative, a poly-N-vinylcarbazole (PVK) derivative, abicarbazole derivative, a polyphenylenevinylene derivative, a polyp-phenylene vinylene (PPV) derivative, a poly(meth)acrylate derivative,a poly(9,9-octylfluorene) derivative, a poly(spiro-fluorene) derivative,TCTA (tris(4-carbazol-9-yl phenyl)amine), TPD(N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine),NPB (N,N′-di(naphthalen-1-yl)-N-N′-diphenyl-benzidine), m-MTDATA(tris(3-methylphenylphenylamino)-triphenylamine), TFB(poly[(9,9-dioctylfluorenyl-2,7-diyl-co(4,4′-(N-4-butylphenyl)diphenylamineD,PFB(poly(9,9-dioctylfluorene)-co-N,N-diphenyl-N,N-di-(p-butylphenyl)-1,4-diaminobenzene),poly-TPD, a metal oxide such as NiO and MoO₃, or a combination thereof,but is not limited thereto.

For example, the hole transport layer 150 may include TCTA and at leastone of the compounds represented by Chemical Formula 1 to ChemicalFormula 9 but is not limited thereto.

The hole transport layer 150 may be formed in a wet coating method suchas spin coating and the like. In this way, the hole transport layer 150may be formed on the emission layer 140 in a relatively simple method.Furthermore, when the emission layer 140 has, e.g., exhibits,hydrophobicity, a polar solvent may be selected as a solvent used forforming the hole transport layer 150 and damage of the emission layer140 may be minimized.

For example, a polymer film such as PPV may be obtained by spin coatinga precursor solution including a PPV precursor polymer and a polarsolvent (e.g., water, methanol, ethanol, etc.) on the emission layer140, thermally treating it, for example, under an inert gas atmosphereof N2 or vacuum at a curing temperature of about 250° C. to about 300°C. for 3 hours to obtain a hole transport layer 150 composed of the PPVthin film.

The hole injection layer 160 may be disposed between the hole transportlayer 150 and the second electrode 170. The hole injection layer 160 maysupply holes into the emission layer 140 together with the holetransport layer 150. However, the hole injection layer 160 may beomitted in consideration of the thicknesses and materials of the holetransport layer 150.

On the other hand, the hole injection layer 160 may be formed of ap-type semiconductor material, or a material doped with a p-type dopantlike the hole transport layer 150. For example, the hole injection layer160 may include a PEDOT (poly (3,4-ethylenedioxythiophene)] derivative,a PSS[poly(styrene sulfonate)) derivative, a poly-N-vinylcarbazole (PVK)derivative, a polyphenylenevinylene derivative, a poly p-phenylenevinylene (PPV) derivative, a polymethacrylate derivative, apoly(9,9-octylfluorene) derivative, a poly(spiro-fluorene) derivative,TCTA (tris(4-carbazol-9-yl phenyl)amine), TPD(N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine),NPB (N,N′-di(naphthalen-1-yl)-N,N′-diphenyl-benzidine), m-MTDATA(tris(3-methylphenyl-N-phenylamino)-triphenylamine), TFB(poly[(9,9-dioctylfluorenyl-2,7-diyl-co(4,4′-(N-4-butylphenyl)diphenylamine]),PFB(poly(9,9-dioctylfluorene)-co-N,N-diphenyl-N,N-di-(p-butylphenyl)-1,4-diaminobenzene),poly-TPD, a metal oxide such as NiO and MoO₃, or a combination thereof,but is not limited thereto.

As described above, in the electroluminescent device 10 according to anembodiment, the electron transport layer 120 includes the inorganicoxide particles 121 and the metal-organic compound or the thermaldecomposition product thereof. Accordingly, the electroluminescentdevice 10 according to an embodiment may minimize void spaces betweenthe neighboring inorganic oxide particles using the metal-organiccompound or the thermal decomposition product thereof. As a result, theelectroluminescent device 10 according to an embodiment may improve theluminous efficiency of the device by decreasing the leakage current.

Hereinafter, a display device including the electroluminescent device 10is described.

A display device according to an embodiment includes a substrate, adriving circuit formed on the substrate, and a first electroluminescentdevice, a second electroluminescent device, and a thirdelectroluminescent device spaced apart from each other in apredetermined interval and disposed on the driving circuit.

The first to third electroluminescent devices have the same structure asthe electroluminescent device 10 and but the wavelengths of the lightsemitted from each quantum dots may be different from each other.

In an embodiment, the first electroluminescent device may be a reddevice emitting red light, the second electroluminescent device may be agreen device emitting green light, and the third electroluminescentdevice may be a blue device emitting blue light. In other words, thefirst to third electroluminescent devices may be pixels expressing,e.g., emitting, red, green, and blue, respectively, in the displaydevice.

However, an embodiment is not necessarily limited thereto, but the firstto third electroluminescent devices may respectively express, e.g.,emit, one of magenta, yellow, cyan, or may express, e.g., emit, othercolors.

On the other hand, one of the first to third electroluminescent devicesmay be the electroluminescent device 10. In this case, the thirdelectroluminescent device expressing, e.g., emitting, blue may bedesirably the electroluminescent device 10.

In the display device according to an embodiment, a hole injectionlayer, a hole transport layer, an electron transport layer, an electroninjection layer, and a hole blocking layer except an emission layer ofeach pixel may be integrated to form a common layer. However, anembodiment is not limited thereto. A hole injection layer, a holetransport layer, an electron transport layer, an electron injectionlayer, and a hole blocking layer may be independently formed in eachpixel of the display device, or a hole injection layer, a hole transportlayer, an electron transport layer, an electron injection layer, a holeblocking layer, or a combination thereof may form a common layer andremaining layers may form a separate independent layer.

The substrate may be a transparent insulating substrate or may include aductile material. The substrate may include a glass or polymer materialand the polymer material may be a material having a glass transitiontemperature (Tg) of greater than about 150° C. For example, thesubstrate may include a COC (cycloolefin copolymer) or COP (cycloolefinpolymer) based material. The first to third electroluminescent devicesmay be formed on the substrate. That is, a substrate of the displaydevice according to an embodiment provides a common layer.

The driving circuit is disposed on the substrate and is independentlyconnected to each of the first to third electroluminescent devices. Thedriving circuit may include a scan line, a data line, a driving powersource line, a common power source line, or a combination thereof, atleast two thin film transistors (TFT) connected to the wire andcorresponding to one organic light emitting diode, and at least onecapacitor, or the like. The driving circuit may have a variety ofsuitable structures.

As described above, a display device according to an embodiment maydecrease a leakage current and improve device characteristics, and thusexcellent electroluminescence characteristics may be exhibited.

Hereinafter, the embodiments are illustrated in more detail withreference to examples. However, these examples are exemplary, and thepresent disclosure is not limited thereto.

EXAMPLE 1

An indium-tin-oxide (ITO) layer is deposited on a glass substrate, and aZnO cluster layer having an average thickness of about 30 nanometers(nm) to 35 nm is formed thereon by spin-coating a composition for afirst electron transport layer which is prepared by dispersing 3millimoles (mmol) of ZnO particles having an average particle diameterof 4.5 nm in ethanol solvent at 4000 rpm for 40 seconds and then, dryingthe composition at 80° C. for 30 minutes.

Subsequently, on the ZnO cluster layer, 0.1 mmol of a zinc-oleatecompound in which Zn and oleate are included in a mole ratio of 1:1 isdissolved in 1 milliliters (mL) of hexane, and then, the solution isspin-coated at 4000 revolutions per minute (rpm) for 40 seconds.

The spin-coated layer is washed by using hexane and then, dried at 80°C. for 30 minutes to remove the hexane to deduce crystallization of thezinc-oleate compound and thus form a layer including a zinc-oleatecompound or a thermal decomposition product thereof on the ZnO clusterlayer. The layer including the zinc-oleate compound or the thermaldecomposition product thereof covers the upper surface of the ZnOcluster layer and has a thinner average thickness than that of the ZnOcluster layer.

Subsequently, on the layer including the zinc-oleate compound or thethermal decomposition product thereof, a blue emission layer having athickness of 25 nm is formed by coating a composition for an emissionlayer, which is prepared by dispersing a blue quantum dot (aZnSeTe/ZnSe/ZnS core-shell blue quantum dot) to which a hydrophobicorganic ligand (oleic acid) is attached and a binder in octane, andthen, drying the composition at 80° C. The blue quantum dot has amaximum light emitting wavelength in a range of 440 nm to 460 nm, a fullwidth at half maximum (FWHM) in a range of 18 nm to 25 nm, and externalquantum efficiency of greater than or equal to 85%.

After forming the blue emission layer, an electron transport layer hasan average thickness (an average thickness sum of the ZnO cluster layerand the layer including the zinc-oleate compound or the thermaldecomposition product thereof) in a range of about 35 nm to 45 nm.

Subsequently, on the blue emission layer, a tris(4-carbazol-9-ylphenyl)amine (TCTA) layer is formed by coating a composition for a holetransport layer, in which TCTA is dissolved in methanol, and then,heating the composition at a high temperature.

Then, a MoO₃ layer as a hole injection layer is formed by depositingMoO₃ on a TCTA layer.

On the MoO₃ layer, a silver (Ag) layer is deposited to manufacture anelectroluminescent device (the ITO/ZnO cluster layer/the layer includingthe zinc-oleate compound or the thermal decomposition productthereof/the blue emission layer/TCTA/MoO₃/Ag) of Example 1.

EXAMPLE 2

An electroluminescent device (an ITO/ZnO cluster layer/a layer includinga zinc-oleate compound or a thermal decomposition product thereof/a redemission layer/Chemical Formula 1/MoO₃/Ag) of Example 2 is manufacturedaccording to the same method as Example 1 except that a red emissionlayer is formed by using a red quantum dot (InP) to which a hydrophobicorganic ligand (oleic acid) is attached instead of the blue quantum dot(ZnSeTe/ZnSe/ZnS) to which a hydrophobic organic ligand (oleic acid) isattached, and a hole transport layer formed of a bicarbazole-basedcompound represented by Chemical Formula 1 is included instead of theTCTA layer.

COMPARATIVE EXAMPLE 1

An electroluminescent device (an ITO/ZnO cluster layer/a blue emissionlayer/TCTA/MoO_(3/)Ag) is manufactured according to the same method asComparative Example 1 except that the blue emission layer is formedright on the ZnO cluster layer.

COMPARATIVE EXAMPLE 2

An electroluminescent device (an ITO/ZnO cluster layer/a layer includinga zinc-acetate compound or a thermal decomposition product thereof/ablue emission layer/TCTA/MoO₃/Ag) is manufactured according to the samemethod as Example 1 except that a composition for a second electrontransport layer is prepared by dissolving 0.1 mmol of a zinc-acetatecompound including Zn and acetate in a mole ratio of 1:1 in 1 mL ofethanol, and a layer including a zinc-acetate compound or a thermaldecomposition product thereof is formed instead of the layer including azinc-oleate compound or a thermal decomposition product thereof.

COMPARATIVE EXAMPLE 3

An electroluminescent device (an ITO/ZnO cluster layer/a ZnCl₂/blueemission layer/TCTA/MoO₃/Ag) is manufactured according to the samemethod as Example 1 except that a ZnCl₂ layer is formed by using acomposition for a second electron transport layer which is prepared bydissolving 0.1 mmol of ZnCl₂ in 1 mL of acetone, instead of the layerincluding a zinc-oleate compound or a thermal decomposition productthereof, and also, using acetone instead of the hexane for the washing.

COMPARATIVE EXAMPLE 4

An electroluminescent device (an ITO/ZnO cluster layer/a layer includinga zincdiethyldithiocarbamate compound or a thermal decomposition productthereof/a blue emission layer/TCTA/MoO₃/Ag) is manufactured according tothe same method as Example 1 except that a layer including azinc-diethyldithiocarbamate compound or a thermal decomposition productthereof instead of the layer including a zinc-oleate compound or athermal decomposition product thereof is formed by using a compositionfor a second electron transport layer prepared by dissolving 0.1 mmol ofa zinc-diethyldithiocarbamate compound including Zn anddiethyldithiocarbamate including in a mole ratio of 1:1 in chloroform(CHCl₃) and also, using chloroform instead of the hexane for thewashing.

COMPARATIVE EXAMPLE 5

An electroluminescent device (an ITO/ZnO cluster layer/a red emissionlayer/Chemical Formula 1/MoO₃/Ag) is manufactured according to the samemethod as Example 2 except that the red emission layer is formed righton the ZnO cluster layer.

Evaluation 1: Comparison of Surface Images between Surfaces of ZnOCluster Layer and ZnO Cluster Layer/Layer Including Zinc-oleate Compoundor Thermal Decomposition Product

FIGS. 5 and 6 respectively show the upper surface of the ZnO clusterlayer in Comparative Example 1 and the surface of the layer including azinc-oleate compound or a thermal decomposition product thereof on theZnO cluster layer in Example 1.

FIG. 5 shows a scanning electron microscopic (SEM) image of the surfaceof the ZnO cluster layer of Comparative Example 1, and FIG. 6 is a SEMimage of the surface of a zinc-oleate or a thermal decomposition productlayer thereof formed on the ZnO cluster layer of Example 1.

Referring to FIG. 5, the ZnO cluster layer of Comparative Example 1 hasa first region (a relatively bright region) filled with ZnO particlesand a second region (a black region) not filled with the ZnO particlesbut left as an empty space, as described above. This second region leftas an empty space includes a void and/or a crack and may work as aleakage path during electron transportation of a device.

On the other hand, referring to FIG. 6, the layer including azinc-oleate compound or a thermal decomposition product thereof ofExample 1 may be formed to cover the ZnO cluster layer as well aspartially and/or fully fill the empty space on the surface of the ZnOcluster layer. Accordingly, when the layer including a zinc-oleatecompound or a thermal decomposition product thereof of the Examples isformed right on the ZnO cluster layer, the void and/or the crack in theZnO cluster layer may be removed or minimized.

Evaluation 2: Device Characteristics of Electroluminescent Devices ofthe Examples and Comparative Examples

Relationships between voltage-current density and luminance-externalquantum efficiency of the electroluminescent devices of Example 1 andComparative Examples 1 to 4 are shown in FIGS. 7 to 8, respectively.

FIG. 7 is a voltage-current density curve of the electroluminescentdevices of Example 1 and Comparative Examples 1 to 4. In FIG. 7, thechange of the current density according to the voltage is represented bya log scale.

Referring to FIG. 7, an electroluminescent device having the electrontransport layer (the layer including a ZnO cluster layer/zinc-oleatecompound or a thermal decomposition product thereof) of Example 1exhibits a current density increase at a voltage of less than or equalto about 2.5 V compared with the electroluminescent devices of theComparative Examples and particularly, a stable current density increaseeffect compared with Comparative Examples 1, and an excellent currentdensity increase effect compared with Comparative Examples 3 to 4.

FIG. 8 is a luminance-external quantum efficiency (EQE) curve showingthe electroluminescent devices of Example 1 and Comparative Examples 1to 4.

Referring to FIG. 8, the electroluminescent device of Example 1 exhibits1.8 times to 2.5 times higher maximum external quantum efficiency thanthose of the Comparative Examples. In addition, when external quantumefficiency is compared depending on a particular luminance, Example 1exhibits about 1.4% of external quantum efficiency at 100 nits (candelasper square meter), Comparative Example 1 exhibits about 0.8% of externalquantum efficiency at 100 nits, Comparative Examples 2 and 4respectively show about 1.0% of external quantum efficiency at 100 nits,and Comparative Example 3 exhibits about 0% of external quantumefficiency at 100 nits.

In addition, as for external quantum efficiency at 500 nits, Example 1exhibits about 2.4%, Comparative Example 1 exhibits about 1.4%,Comparative Example 2 exhibits about 1.5%, Comparative Example 3exhibits 0%, and Comparative Example 4 exhibits about 0.9%, and as forexternal quantum efficiency at 1000 nits, Example 1 exhibits about 2.3%,Comparative Example 1 exhibits about 1.2%, Comparative Example 2exhibits about 0.5%, and Comparative Examples 3 and 4 respectively showabout 0%.

On the other hand, as for maximum luminance of each electroluminescentdevice, Example 1 exhibits about 1400 Cd/m², Comparative Example 1exhibits about 1200 Cd/m², Comparative Example 2 exhibits about 1000Cd/m², Comparative Example 3 exhibits about 24 Cd/m², and ComparativeExample 4 exhibits about 740 Cd/m².

First, Comparative Example 3 exhibits deteriorated properties relativeto the other Comparative Examples. While not wanting to be bound bytheory, it is understood that he reason that Comparative Example 3exhibits deteriorated properties relative to the other ComparativeExamples is that when ZnCl₂ instead of the metal-organic compound isincluded as a compound for forming the electron transport layer, theZnCl₂ layer on the ZnO cluster layer has equivalent or more voids and/orcracks than the ZnO cluster layer. Accordingly, when a second electrontransport layer is formed by using not a metal-organic compound but aninorganic compound such as ZnCl₂ and the like, device characteristicsare more difficult to realize.

On the other hand, Comparative Examples 2 to 4 using a polar solventduring formation of the layer including a metal-organic compound or athermal decomposition product thereof show equivalent external quantumefficiency and/or maximum luminance (Comparative Example 2) to those ofComparative

Example 1 having only a ZnO cluster layer and much deteriorated externalquantum efficiency and/or maximum luminance (Comparative Examples 3 and4) compared with those of Comparative Example 1. Accordingly, while notwanting to be bound by theory, it is understood that when ametal-organic compound having solubility in a non-polar solvent isdeposited using a polar solvent instead, the polar solvent may have aninfluence on the ZnO cluster layer.

In other words, Comparative Examples 2 to 4 fills a void and/or a crackof a ZnO cluster layer by using a metal-organic compound or a thermaldecomposition product thereof similarly to Example 1 but with a polarsolvent, which results in damage and the like on the ZnO cluster layerby the polar solvent, and accordingly, the metal-organic compound or thethermal decomposition product thereof has no significant effect(Comparative Example 2) and rather exhibits deteriorated performance(Comparative Examples 3 and 4).

Accordingly, when a layer including a metal-organic compound or athermal decomposition product thereof on a ZnO cluster layer is formedby using a metal-organic compound having solubility in a non-polarsolvent, and a non-polar solvent is used, Example 1 exhibits excellentexternal quantum efficiency over about 100 nits to 1000 nits comparedwith Comparative Examples.

Evaluation 3: Life-span Characteristics of Electroluminescent Devices ofthe Examples and Comparative Examples

Life-span characteristics of the electroluminescent devices (blue) ofExample 1 and Comparative Example 1 and life-span characteristics of theelectroluminescent devices (red) of Example 2 and Comparative Example 5are shown in FIGS. 9 to 10, respectively.

FIG. 9 shows life-span characteristics of the electroluminescent devicesof Example 1 and Comparative Example 1 and FIG. 10 shows life-spancharacteristics of the electroluminescent devices of Example 2 andComparative Example 5.

Referring to FIGS. 9 and 10, since Example 1 exhibits a luminance of 50%(T50) of about 1.5 hours, while Comparative Example 1 exhibits T50 ofabout 1 hour, Example 1 exhibits an about 1.5 times life-spanimprovement effect with a reference to that of a blue electroluminescentdevice. On the other hand, Example 2 exhibits T50 of about 67.7 hours,while Comparative Example 5 exhibits T50 of about 9.7 hours, and thus anabout 7 times life-span improvement effect with a reference to a redelectroluminescent device.

Accordingly, the electroluminescent devices of the Examples exhibitexcellent life-span characteristics compared with the ComparativeExamples, which did not include a layer of a metal-organic compound or athermal decomposition product thereof. While not wanting to be bound bytheory, it is understood that the reason the electroluminescent devicesof the Examples exhibit excellent life-span characteristics comparedwith the Comparative Examples is that the layer of a metal-organiccompound or a thermal decomposition product thereof makes possible astable current injection into an electron transport layer and thusimproves a charge carrier balance.

While this disclosure has been described in connection with what ispresently considered to be practical example embodiments, it is to beunderstood that the invention is not limited to the disclosedembodiments. On the contrary, it is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

<Description of Symbols> 10: electroluminescent device 100: substrate110: first electrode 120: electron transport layer 121: inorganic oxideparticle 124: first layer 126: second layer 140: emission layer 141:light emitting particle 150: hole transport layer 160: hole injectionlayer 170: second electrode

What is claimed is:
 1. An electroluminescent device, comprising a firstelectrode and a second electrode facing each other; an emission layerdisposed between the first electrode and the second electrode, theemission layer comprising light emitting particles; an electrontransport layer disposed between the first electrode and the emissionlayer; and a hole transport layer disposed between the second electrodeand the emission layer, wherein the electron transport layer comprisesinorganic oxide particles and a metal-organic compound, themetal-organic compound or a thermal decomposition product of themetal-organic compound being soluble in a non-polar solvent.
 2. Theelectroluminescent device of claim 1, wherein the metal-organic compoundcomprises Zn, Ti, Sn, W, Ta, Mg, Ga, Na, Cu, Ag, or a combinationthereof.
 3. The electroluminescent device of claim 1, wherein themetal-organic compound comprises an oleate compound, an amine compound,or a combination thereof.
 4. The electroluminescent device of claim 1,wherein the non-polar solvent comprises a C6 to C20 linear, branched, orcyclic aliphatic hydrocarbon, a C6 to C20 aromatic hydrocarbon,methylene chloride, ethyl acetate, or a combination thereof.
 5. Theelectroluminescent device of claim 1, wherein the inorganic oxideparticles comprise zinc, titanium, zirconium, tin, tungsten, tantalum,magnesium, gallium, sodium, copper, silver, or a combination thereof. 6.The electroluminescent device of claim 1, wherein the inorganic oxideparticles are dispersible in a polar solvent.
 7. The electroluminescentdevice of claim 1, wherein a surface of the electron transport layerfacing the emission layer comprises a first portion in which theinorganic oxide particles are disposed and a second portion in which themetal-organic compound or the thermal decomposition product of themetal-organic compound is between neighboring inorganic oxide particles.8. The electroluminescent device of claim 1, wherein the electrontransport layer comprises a first layer comprising the inorganic oxideparticles and a second layer comprising the metal-organic compound orthe thermal decomposition product of the metal-organic compound.
 9. Theelectroluminescent device of claim 8, wherein the second layer isdisposed between the first layer and the emission layer.
 10. Theelectroluminescent device of claim 8, wherein the first layer comprisesa cluster layer comprising the inorganic oxide particles.
 11. Theelectroluminescent device of claim 10, wherein a surface of the firstlayer facing the second layer comprises the metal-organic compound orthe thermal decomposition product of the metal-organic compound.
 12. Theelectroluminescent device of claim 8, wherein the first layer comprisesan organic matrix filled in between neighboring inorganic oxideparticles and the organic matrix comprises an organic material that isdifferent from the metal-organic compound or the thermal decompositionproduct of the metal-organic compound.
 13. The electroluminescent deviceof claim 8, wherein an average thickness of the second layer is lessthan an average thickness of the first layer.
 14. The electroluminescentdevice of claim 1, wherein the inorganic oxide particles comprise metaloxide particles, and a metal in the metal oxide particles and a metal inthe metal-organic compound are the same.
 15. The electroluminescentdevice of claim 1, wherein an average thickness of the electrontransport layer is about 20 nanometers to about 60 nanometers.
 16. Theelectroluminescent device of claim 1, wherein the light emittingparticles comprise quantum dots.
 17. The electroluminescent device ofclaim 1, wherein the light emitting particles have a core-shellstructure.
 18. The electroluminescent device of claim 1, wherein thelight emitting particles comprise a Group II-VI compound that does notcomprise Cd, a Group III-V compound, a Group IV-VI compound, a Group IVelement or compound, a Group compound, a Group I-II-IV-VI compound thatdoes not comprise Cd, or a combination thereof.
 19. Theelectroluminescent device of claim 1, wherein a hydrophobic organicligand is attached to a surface of the light emitting particles.
 20. Adisplay device comprising the electroluminescent device of claim
 1. 21.A method of forming an electroluminescent device, the method comprisingdisposing a first electrode and a second electrode facing each other;disposing an emission layer between the first electrode and the secondelectrode, the emission layer comprising light emitting particles;disposing an electron transport layer between the first electrode andthe emission layer, the electron transport layer comprising inorganicoxide particles, and a metal-organic compound, the metal-organiccompound or a thermal decomposition product of the metal-organiccompound being soluble in a non-polar solvent; and disposing a holetransport layer between the second electrode and the emission layer.